An industrial process that requires considerable pressurization of solutions is the desalting of seawater or other relatively highly concentrated saline aqueous solutions using reverse osmosis membrane technology. In this type of process, the saline solution is pressurized, by using a mechanical or electrical driven device, to reach a pressure that exceeds the osmotic pressure of the solution, which is a function of the solutes' concentration. The excess pressure, which is equivalent to the working pressure less the osmotic pressure of the pressurized solution, is the driving force that causes the transport of pure water through the reverse osmosis membrane, thus producing depressurized desalinated water on the other side of the membrane at atmospheric pressure. The transport of pure water through the membrane causes an increase in the solutes' concentration in the pressurized solution to a point that the osmotic pressure becomes too high, thus causing the driving pressure, which effects the transport of pure water across the membrane, to diminish. This requires depressurization and draining of the concentrated solution and replacement of the drained solution with newly pressurized feed solution.
Under steady-state continues operation of a desalination process, a feed solution is continuously pressurized into the reverse osmosis membrane module vessel using an electrically or mechanically driven high pressure pump, and the concentrated exhaust solution is depressurized through a valve, in which case, no useful energy is recovered, or through a turbine, which helps recover a portion of the hydraulic energy contained in the exhaust solution. The economy with which the reverse osmosis desalination process can be practiced is directly dependent upon the efficiency with which the energy input to the process during the pressurization of the feed solution can be recovered from the pressurized exhaust solution after separation of the pure water.
Several devices have been proposed and a few machines are available on the market to recover substantial portions of the energy contained in an exhaust pressurized fluid system. Examples of the available machines are the reverse centrifugal pumps and the hydraulic turbine in which the pressurized exhaust fluid is used to drive the machine, thus generating mechanical energy, which is then utilized to assist and reduce the energy required for driving the main pressurizing device in the process. These engines have reasonable reliability, however, the efficiency with which these machines recover the hydraulic energy contained in the pressurized exhaust fluid is in the range of 60% to 70% and may be, up to about 80%. The actual useful energy, which can then be transferred to the fresh feed fluid that is being pressurized, will be substantially less than the amount of energy recovered by these machines because it has to be transferred through a pumping device.
Other devices that are still in the development stage and not yet widely used commercially are of the type known as flow work exchangers, or pressure exchangers, which can be utilized to simultaneously depressurize the exhaust fluid and pressurize the feed fluid in the process. One type of exchanger is generally based on a piston reciprocating in a cylinder, where, in one stroke, the pressurized exhaust fluid is admitted at one end of the cylinder applying its pressure on one side of the piston causing it to move towards the other end of the cylinder, thus pressurizing the feed fluid that had previously filled the cylinder while, in the return stroke, the feed fluid is admitted at the second end of the cylinder at relatively low pressure causing the piston to move back towards the first end of the cylinder, hence, discharging the exhaust fluid, which had filled the cylinder during the previous stroke, at normal atmospheric pressure. The successive alternation between high and low pressure filling strokes requires rapidly operating valves in each pair of the liquids' inlet and outlet ports into and from the cylinder, respectively, in which the piston reciprocates. Due to the requirement for the inclusion of a piston-cylinder arrangement as well as valves, energy losses may be high. Moreover, there may be practical limitations on the use of rapidly operated valves in handling large volumes of liquids.
Another type of pressure exchanger is based on a cylindrical rotor slidingly moving at its ends and at its circumferential surface against matching surfaces of a stationary structure, i.e., a matching stator. The rotor has longitudinal conduits that are circumferentially spaced from each other and each has an opening located in the sliding end surfaces of the moving rotor. The stator, which is in contact with the rotor, has two matching sealing flat surfaces that slidingly and sealingly engage end surfaces of the cylindrical rotor in which the end openings of the longitudinal conduits are located. The stator further has one pair of fluid inlet passageways extending into it and each opens at one of the sealing surfaces. Another pair of fluid discharge passageways are spaced from the fluid inlet passageways, and also opens at one of the sealing surfaces of the stator. The openings of the longitudinal conduit in the moving rotor and the openings of the passageways in the stator are positioned in their respective surfaces in such a way that the rotor is moved with its end surfaces in which the longitudinal conduits' openings are located in sealing contact with the sealing surfaces of the stator. The openings into the two ends of each longitudinal conduit are periodically brought into communication with a fluid inlet passageway at one end of the longitudinal conduit and fluid discharge passageway at the other end of the longitudinal conduit, and alternately, the position of the fluid inlet and fluid discharge passageways with respect to the ends of the longitudinal conduit are periodically reversed. These types of machines require a substantial amount of energy for moving and constantly maintaining the movement of the large mass of the rotor, and also to overcome the friction, which is caused by the relative motion between the large area of the sealingly sliding surfaces of the rotor against the matching surfaces of the stator. Moreover, a substantial portion of the volume of the longitudinal conduits is a dead volume that is filled with liquid reciprocating between the two ends of the longitudinal conduits separating the incoming liquid from the outgoing liquid to minimize the mixing between the two working liquids.